The present invention relates to an optical printer having a print head made up of a number of light emitting elements and capable of reproducing medium densities which represent the tones of a document faithfully. More particularly, the present invention is concerned with an optical printer operable at high speed with simple circuitry and control.
A recent achievement in the imaging art is an optical printer having a print head which is implemented as an array of numerous light emitting elements such as minute light emitting diodes (LEDs). The LEDs are selectively turned on or off in response to a two-level digital image signal which is representative of the density of a document image, thereby exposing the surface of a photoconductive element. A latent image electrostatically formed on the photoconductive element by the exposure is turned to a visible image by development. The visible image is transferred into a paper sheet and then fixed on the paper sheet. This type of printer has only a small number of mechanically movable parts and, therefore, can print out images at high speed and with a minimum of noise. For this reason, many modern facsimile machines, computer terminals and other similar equipment use an optical printer. Of course, this type of printer has to reproduce medium densities faithfully. To meet this requirement, it has been customary to use a dither method. The dither method uses a dither matrix for dividing a document image into pixels, as is well known in the art. The density of each pixel is compared with associated one of predetermined threshold values of the dither matrix and is thereby determined to be white or black. The aggregation of the resulting small dots renders medium tones of an image. The number of black dots is varied on the basis of the densities of the individual pixels, whereby the densities of a multi-tone image is rendered. The problem with the dither method is that when the number of such small dots is small, the resolution is lowered to prevent a reproduced image from bearing clear medium tones.
In the light of the above, it has been proposed to render the densities of individual pixels by the 2.sup.n -tone intensity modulation of individual light emitting elements for an n-bit digital image signal. For example, Japanese Patent Laid-Open Publication No. 127467/1984 discloses a multi-tone printer driving device adopting a so-called dynamic light emitting element turn-on control for the intensity modulation. Specifically, resistors are connected in series with light emitting elements, and their values are switched over to control the drive currents to be applied to the associated light emitting elements. As a result, the intensity of light issuing from each light emitting element and, therefore, the intensity of exposure of a photoconductive element is controlled. On the other hand, Japanese Patent Laid-Open Publication No. 64870/1985 implements the dynamic light emitting element turn-on control by controlling the turn-on time of light emitting elements in matching relation to the image density. Specifically, this prior art uses LED drivers associated one-to-one with light emitting diodes, a gate circuit for controlling the turn-on time of each light emitting element, a data selector for selecting light emitting elements to be turned on, a turn-on time control signal generating circuit for generating a signal for controlling the gate circuit to change the turn-on time, and a gate circuit for ANDing the turn-on time control signal and a turn-on element selection signal. In such a configuration, the densities of a document image are determined with respect to a plurality of steps, and the turn-on time of the light emitting elements is variably controlled on the basis of the result of decision.
However, a drawback with the prior art schemes described above is that they need complicated circuitry and, therefore, fail to cut down the cost of the apparatus. Specifically, previously mentioned Laid-Open Publication No. 127467/1984 needs the same number of light emitting element drivers as the tones to be rendered. This, coupled with the fact that it has some problems concerning high-speed operations, makes the circuit arrangement extremely complicated. The other Laid-Open Publication No. 64870/1985 mentioned specifically cannot implement a high-speed printer unless all the light emitting elements are provided with exclusive turn-on time control signal generating circuits, whereby the circuitry becomes more complicated as the number of light emitting elements of light emitting element blocks increases.